Redox flow secondary batteries are to store and discharge electricity, and belong to large-size stationary batteries used for leveling the amounts of electricity used. The redox flow secondary battery is configured such that a positive electrode and an electrolyte solution containing a positive electrode active substance (positive electrode cell) and a negative electrode and a negative electrode electrolyte solution containing a negative electrode active substance (negative electrode cell) are separated by a separation membrane; charge and discharge are carried out by utilizing the oxidation and reduction reactions of both the active substances; and the electrolyte solutions including both the active substances are circulated from storage tanks to an electrolytic bath, and a current is taken out and utilized.
As an active substance contained in an electrolyte solution, there are used, for example, iron-chromium-based ones, chromium-bromine-based ones, zinc-bromine-based ones, and vanadium-based ones utilizing the difference in electric charge.
Particularly, vanadium-type secondary batteries, since having advantages of a high electromotive force, a high electrode reaction rate of vanadium ions, only a small amount of hydrogen generated as a side-reaction, a high output, and the like, are being developed earnestly.
For separation membranes, devices are made so that electrolyte solutions containing active substances of both electrodes are not mixed. However, conventional separation membranes are liable to be oxidized and for example a problem thereof is that the electric resistance needs to be made sufficiently low. In order to raise the current efficiency, an ion-exchange membrane is demanded which is excellent in the ion permselectivity preventing the mutual permeation of each active substance ion contained in each cell electrolyte solution (contamination with electrolytes in electrolyte solutions of both electrodes) as much as possible, and allowing sufficiently easy permeation of protons (H+) carrying the charge.
The vanadium-type secondary battery utilizes an oxidation and reduction reaction of divalent vanadium (V2+)/trivalent vanadium (V3+) in a negative electrode cell, and oxidation and reduction reaction of tetravalent vanadium (V4+)/pentavalent vanadium (V5+) in a positive electrode cell. Therefore, since electrolyte solutions of the positive electrode cell and the negative electrode cell have ion species of the same metal, even if the electrolyte solutions are permeated through a separation membrane and mixed, the ion species are normally reproduced by charging; therefore, there hardly arises a large problem as compared with other metal species. However, since active substances becoming useless increase and the current efficiency decreases, it is preferable that the active substance ions freely permeate as little as possible.
There are conventionally batteries utilizing various types of separation membranes (hereinafter, also referred to as “electrolyte membrane” or simply “membrane”); and for example, batteries are reported which use porous membranes allowing free permeation by an ionic differential pressure and an osmotic pressure of electrolyte solutions as the driving force. For example, Patent Literature 1 discloses as such a porous membrane a polytetrafluoroethylene (hereinafter, also referred to as “PTFE”) porous membrane, a polyolefin (hereinafter, also referred to as “PO”)-based porous membrane, a PO-based nonwoven fabric, and the like.
Patent Literature 2 discloses a composite membrane in combination of a porous membrane and a hydrous polymer so that both electrolyte solutions do not move by a pressure difference between cells.
Patent Literature 3 discloses the utilization of a membrane of a cellulose or an ethylene-vinyl alcohol copolymer as a nonporous hydrophilic polymer membrane having a hydrophilic hydroxyl group.
Patent Literature 4 states that the utilization of a polysulfone-based membrane (anion-exchange membrane) as a hydrocarbon-based ion-exchange resin makes the current efficiency thereof 80% to 88.5% and the radical oxidation resistance excellent.
Patent Literature 5 discloses a method of using a fluorine-based or a polysulfone-based ion-exchange membrane as a separation membrane and making expensive platinum to be carried on a porous carbon of a positive electrode in order to raise the current efficiency, to thereby raise the reaction efficiency.
Patent Literature 6 discloses an iron-chromium-type redox flow battery in which a hydrophilic resin is coated on pores of a porous membrane of a polypropylene (hereinafter, also referred to as “PP”) or the like. An Example of the Patent Literature uses a membrane covered in a thickness of several micrometers with a fluorine-based ion-exchange resin (made by Du Pont K.K., registered trademark: “Nafion”) on both surfaces of a PP porous membrane of 100 μm in thickness. Here, Nafion is a copolymer containing a repeating unit represented by —(CF2—CF2)— and a repeating unit represented by —(CF2—CF(—O—(CF2CFXO)n—(CF2)m—SO3H))— wherein X=CF3, n=1, and m=2.
Patent Literature 7 discloses an example of a vanadium-type battery decreased in the cell electric resistance as much as possible and raised in the efficiency by the improvement of the electrode sides using of a two-layer liquid-permeable porous carbon electrode having a specific surface grating.
Patent Literature 8 discloses an example of a vanadium-type redox flow battery using a separation membrane which is low in the resistance, is excellent in the proton permeability, is of an anion-exchange type having a pyridinium group (utilizing N+ of a cation), and contains a crosslinked polymer obtained by copolymerizing a styrene-based monomer and divinylbenzene.
Patent Literature 9 discloses an example of improving the ion permselectivity by utilizing a membrane having a structure in which a cation-exchange membrane (a fluorine-based polymer or another hydrocarbon-based polymer) and an anion-exchange membrane (a polysulfone-based polymer) are alternately laminated, and disposing a cation-exchange membrane on the side of a positive electrode electrolyte solution.
Patent Literature 10 discloses an example of using an anion-exchange membrane, as a membrane excellent in the chemical resistance, low in the resistance, and excellent in the ion permselectivity, obtained by compositing a porous base material composed of a porous PTFE-based resin with a crosslinked polymer having a repeating unit of a vinyl heterocyclic compound having two or more hydrophilic groups (vinylpyrrolidone having an amino group, or the like). The principle described therein is that although when a potential difference is applied on metal cations having a large ion diameter and a much amount of electric charge, the membrane permeation of the metal cations is inhibited due to the electric repulsion by cations of the respective separation membrane surfaces, protons (H+), having a small ion diameter and being monovalent can easily diffuse in and permeate the separation membrane having cations to thereby give a low electric resistance.